The brain waves of a human subject provide an electrical signal at the microvolt level. It is possible by connecting removable electrode leads to the scalp of the subject to detect and amplify those faint electrical signals. In the past, those electrical signals were amplified and recorded on a paper strip chart showing an analog wavy line for each recording channel in an EEG (electroencephalograph). EEG has been used for many medical and testing purposes, for example, the testing of auditory, visual, somatosensory and motor systems and testing for pathological brain dysfunction. Presently there are available EEG instruments which can simultaneously detect the faint signals and convert the signals to digital data for recording and analysis.
However, a major problem in obtaining an accurate recording of brain waves has been "movement artifact", which consists of electrical contaminants arising as a consequence of the subject's muscle movements. For example, when a subject moves her head, the muscle action may result in a contaminating electrical signal that will distort or drown out the brain signal. Movement artifacts are caused by the relative movement of the electrode with respect to the scalp. The changes in contact of the electrode on the head result in changes in the impedance and induced potentials at the electrode-scalp interface, producing movement artifacts in the measured EEG. Also head movements can cause the electrode wires to generate a current as they move through the ambient 60 Hz magnetic field, producing "electrode sway artifact".
In addition to movement artifacts, "eye artifact" is another source of electrical contaminants in EEG recordings. When the eyes move or the eyelids blink, electrical potentials can be recorded from electrodes located near the eyes: the "electro-oculogram" or EOG. The eye related potential can also be picked up by EEG electrodes resulting in contaminated EEG signals. Sometimes movement and eye artifacts are present in the measured EEG signal simultaneously.
Because of contamination by movement and eye artifacts, subjects must sit still and restrain eye movements during an EEG recording to minimize head, body or eye movement artifacts. In a clinical situation this is particularly problematic in recording from children, from ambulatory patients, and from patients who are monitored for long periods of time, such as those in intensive care units or in epileptic seizure monitoring diagnostic clinics. Movement and eye artifacts are also a major obstacle hindering recordings of brain electrical activity in laboratory or real world situations, where subjects are performing a task that may involve head, body and eye movements, for instance, working at a computer or flying an airplane. Correction algorithms have been available for eye artifacts Barlow 1986! but not for movement or compound movement and eye artifacts. Currently data contaminated by head or body movement artifacts, with or without eye artifacts, must be discarded, resulting in prolonged recording sessions or the complete inability to monitor brain activity Gevins, et al, 1977!. Such discarding of data or marking of contaminated data is mentioned in U.S. Pat. No. 4,171,696 to John and in U.S. Pat. No. 4,736,751 to Gevins, Morgan and Greer.
In U.S. Pat. No. 4,913,152 to Ko et al, magnetoencephalography (MEG) is used to localize and track neuromagnetic signals. In FIG. 4 of the Ko et al patent, auxiliary sensors 24 are used "to help reduce noise contamination". One of the suggested auxiliary sensors is an accelerometer "to detect sensor motion or vibration".
U.S. Pat. No. 4,417,592 to John, for an EEG system, mentions the use of "accelerometer channels, the accelerometer being attached to the patient's head."
These prior patents to Ko et al and to John, incorporated by reference herein, do not disclose the use of an adaptive interference canceler connected to the outputs of the EEG electrodes, accelerometer and eye monitoring channels. The implication in these patents is that if the accelerometer detects movement, the portion of the recording contaminated by such movement would be discarded.